180 



1819, to the unwearied activity of Edward Sabine, who, after 

 having observed the oscillations of the same needles at the 

 American north pole, in Greenland, at Spitzbergen, and on the 



des magnet. Vereim, 1838, s. 36-39 ; Ennan, Physical. Beob., 1841, 

 s. 629-579), in England (Sabine, Report on Magnet. Intensity, 1838, 

 p. 43-62 ; Contributions to Terrestrial Magnetism, 1843), and in 

 France (Becquerel, Traite de Elec.tr. et de Magnet., t. vii. p. 354-367), 

 to reduce the oscillations observed in any part of the Earth to the stand- 

 ard of force which I found on the magnetic equator in Northern Peru ; 

 so that, according to the unit thus arbitrarily assumed, the intensity of 

 the magnetic force at Paris is put down as 1*348. The observations 

 made by Lamanon in the unfortunate expedition of La Perouse, during 

 the stay at Teneriffe (1785), and on the voyage to Macao (1787), are still 

 older than those of Admiral Rossel. They were sent to the Academy of 

 Sciences, and it is known that they were in the possession of Condorcet 

 in the July of 1787 (Becquerel, t. vii. p. 320) ; but notwithstanding the 

 most careful search, they are not now to be found. From a copy of a 

 very important letter of Lamanon, now in the possession of Captain 

 Duperrey, which was addressed to the then perpetual secretary of the 

 Academy of Sciences, but was omitted in the narrative of the Voyage 

 de La Perouse, it is stated "that the attractive force of the magnet 

 is less in the tropics than when we approach the poles, and that the 

 magnetic intensity deduced from the number of oscillations of the needle 

 of the inclination-compass varies and increases with the latitude." 

 If the Academicians, while they continued to expect the return of the 

 unfortunate La Perouse, had felt themselves justified, in the course of 

 1787, in publishing a truth which had been independently discovered 

 by no less than three different travellers, the theory of terrestrial mag- 

 netism would have been extended by the knowledge of a new class of 

 observations, dating eighteen years earlier than they now do. This 

 simple statement of facts may probably justify the observations contained 

 in the third volume of my Relation liistorique (p. 615) : " The obser- 

 vations on the variation of terrestrial magnetism, to which I have 

 devoted myself for thirty-two years, by means of instruments which 

 admit of comparison with one another, in America, Europe, and Asia, 

 embrace an area extending over 188 degrees of longitude, from the 

 frontier of Chinese Dzoungarie to the west of the South Sea bathing the 

 coasts of Mexico and Peru, and reaching from 60 north lat. to 12 south 

 lat. I regard the discovery of the law of the decrement of magnetic force 

 from the pole to the equator, as the most important result of my American 

 voyage." Although not absolutely certain, it is very probable that Con- 

 dorcet read Lamanon's letter of July 1787 at a meeting of the Paris 

 Academy of Sciences ; and such a simple reading 1 regard as a sufficient 

 act of publication. (Annuaire du Bureau des Longitudes, 1842, p. 463.) 

 The first recognition'of the law belongs, therefore, beyond all question, 

 to the companion of La Perouse ; but long disregarded or forgotten, the 

 krnwledge of tae law that the intensity of the magnetic force of the 



